Haulin_in_Dixie

The last few months my engine on pull will sometimes sound rough, although the power is great. Lately as I put in another thread I have picked up a miss at idle, sometimes. I cured mine, other should check. I have been aware that my tank operates under a slight vacuum and have cleaned the vent a couple of times. The 1/4&quot; line does not vent the tank! I disassembled the vent unit and removed the restrictor from the top of it. The miss is gone, the engine is smooth again. The tank has no vacuum now.<br><br>There are two valves in the vent unit. Dot requires the check ball so that if you flip the fuel does not run out. That is on the bottom of the vent unit. There is a restrictor above that which restricts the venting and gets dirty. On a good day there is little vacuum but once dirty there is a lot. That is the vent with the 1/8 inch orfice. you also need to connect a short piece of vacuum hose to it to keep water and dirt out. This made a big difference on mine, wonder if this is a cause for lift pumps going out. If it restricts the venting after a long run enough to cause missing, it sure cannot be good for the system. Modifying it requires breaking the top out and gluing a cover on it, then you have positive venting and retain the spill check system.

Desert Diesel

This sounds like what my truck has been doing for about a month now. I'm going to check in this a little further... Thanks!<br><br>--Phillip<br>

admin

Is there any way you might be able to take some pictures of this and post them?

Desert Diesel

Not sure who that was directed at, but I'll be sure to document (in text, and photo) the whole process, and my findings

Haulin_in_Dixie

[quote author=Jack Thorpe link=board=4;threadid=6866;start=0#65692 date=1037119398]<br>Is there any way you might be able to take some pictures of this and post them?<br>[/quote]<br><br>Jack I did the mod in the middle of the night but can take a couple of pictures and with the aid of a drawing can show what I did. Give me a couple of days and I will post them. I also have to post the pics of the dual wheel conversion as I promased. I did take the pics, just have to post them.<br><br>To check to see if there is a difference just leave the cap loose for a while. Before I removed the bed I did that for quite a while, but without the bed the cap would be lost.

j

pictures would be a big help. just reading it i cant picture where it is. i cant say it enough &quot;THIS BOARD RULES&quot;

BigBlue

I can't picture it either. Sounds like a resonable explanation to the lift pump problem. Probably not the only reason, but could be one of them.

AlpineRAM

Tried the fuel cap open way of driving- wow, my fuel gauge starts making sense and the idle is much better. <br>HID- would love to see some explanation.<br><br>AlpineRAM

Haulin_in_Dixie

Ok I will try to make some sense here. I did not take pictures when I did the mod. Then a made a drawing and went to scan it in. The scanner went south. So I have taken a picture of the drawing with my digital, not as clear as I wanted but the best that I could do. Ok on a fuel tank there has to be a vent to relieve the vacuum when the fuel is pumped out. DOT regulations require and for safety purposes it has to have a positive pressure check valve to keep the fuel from running out if you flip the truck. On steel tanks it also keeps fuel from spraying out if it gets too hot as in a fire on the truck.

On the top of the ’99 tank (and all of the later models that I have seen) all lines and the fuel level gage are housed on one unit about six inches around. On the top of this there is a ¼ inch line that is plugged and points straight up. Uncapping this one does not seem to vent the tank, so better left plugged. It would be good for a return line or possibly to add fuel from another tank. The actual vent system is about two inches around with a horizontal 1/8 fitting on it for a small hose. There is no hose attached and probably that is the problem. All the road spray is able to get to it easily. You can pry the vent unit out with a screwdriver. The rubber grommet will come out with it. Remove the rubber from the vent and put it back on the tank. When reinstalling the vent, the vent just pushes into the rubber, sealing the tank.

On the bottom of the vent unit you can pry the fingers apart that hold in a piston with a spring on it. The top of the piston has a rubber flap that will seal off the vent in case of an upset. This is the needed check valve to keep fuel from running out. With the piston and spring out of the unit you can see a small hole at the top on the inside. Between the vent orifice on the outside and the small hole on the inside there is a small disk that almost seals off the vent and somehow is supposed to make a sort of regulator, I would guess to keep some of the water and trash out of the tank. This is what gets dirty and at best provides a poor vent.

I used a drill through the small hole in the bottom and with a small hammer drive it up which broke out the white plug in the top at the vent orifice. Taking out the small disk I glued (silicone) a small plastic piece back over the hole to seal the unit back up. Reinstalled the vent unit and then added a piece of vacuum hose about eight inches long which just hangs down beside the tank. Water cannot get into the tank because water will run down from gravity. On a pickup you could get fancy and put a longer hose on it, maybe up the front of the pu bed and add a vent cap like the rear axle uses, but the short hose is what I needed.

Once I realized that the tank was running under a vacuum I knew that I had to eliminate that. It changed my idle and the truck pulls much smoother under a hard pull. I cannot tell you of a gage difference because I also put a new gage regulator in at the same time. Basically the tank needs a positive vent, this was my way of curing the problem. There can be other ways, for instance a vented cap. I liked keeping the check valve on the tank as it was designed. My thoughts on this are that if the change affects the running at all, than the injector pump has been starving for fuel. No wonder the lift pumps go bad. I also cannot say that any other truck has a bad vent, but mine did.

admin

Hmmm, this is definately something I'm going to look at today. This is a really cool thread!

j

NOW it all makes sense to me. thanks for the thread, i too am going to ck this asap.

AlpineRAM

HID- great explanation. Have been running a tank with the fillercap cracked open. With the fillercap tight the fuel gauge would read full for some 150 miles and then drop. Now it works as designed. The idle is much smoother, and the truck pulls better. FP is up again. ( Still don't know why this is the case, because the return lines go back to the tank, so it shouldn't make a difference- maybe there ara some soft rubber lines in there that collapse and form a restriction when there is a vacuum in the tank ??? )<br>I will mount a hose to the gutted vent and think of letting the hose got to the air inlet- maybe post turbo with a pressue regulator. (set to 2psi or so) -this would be a positive vent with clean air.<br><br>AlpineRAM

admin

I have been running mine today with the filler cap cracked as well. I haven't noticed a difference in idle, or in fuel pressure at the moment. I am also going to run through the whole tank of fuel this way because I would like to see what happens. My truck will also show full for a very long time before the gauge finally begins to move. then it drops like a rock.

Haulin_in_Dixie

[quote author=AlpineRAM link=board=4;threadid=6866;start=0#66825 date=1037304145]<br>HID- great explanation. Have been running a tank with the fillercap cracked open. With the fillercap tight the fuel gauge would read full for some 150 miles and then drop. Now it works as designed. The idle is much smoother, and the truck pulls better. FP is up again. ( Still don't know why this is the case, because the return lines go back to the tank, so it shouldn't make a difference- maybe there ara some soft rubber lines in there that collapse and form a restriction when there is a vacuum in the tank ??? )<br>I will mount a hose to the gutted vent and think of letting the hose got to the air inlet- maybe post turbo with a pressue regulator. (set to 2psi or so) -this would be a positive vent with clean air.<br><br>AlpineRAM<br>[/quote]<br><br>Alpine, air has to replace the volume of fuel burned or the tank will go into vacuum, with the lift pump pulling against the vacuum.

AlpineRAM

HID: I'd thought a bit . I shouldn't matter for filling the fuel filter, but it shure would matter on feeding the VP44. <br>Anyway- the practical test with the fillercap shows that you are right. <br>Maybe the vacuum in the tank leads to problems like loosing prime on older vehicles? Lots of threads where you find cracked return lines that make you loose prime. <br>What do you think about pressurizing the tank? <br><br>AlpineRAM